The present invention relates to a rechargeable electrochemical cell with a hydrogen electrode as a negative electrode. More particularly, the invention relates to a electrochemical cell which is suitable as a secondary cell and in which a hydrogen negative electrode is comprised by a hydrogen absorbing alloy including rare earth elements as constituents.
There is, by way of a rechargeable electrochemical cell comprising hydrogen absorbing alloy as a negative electrode, a sealed secondary cell with a negative electrode constituted by LaNi.sub.5 such as disclosed in U.S. Pat. No. 3,874,928, for example. However, although use of the element La as the rare earth component of the hydrogen absorbing alloy used for the negative electrode offers the promise of superior secondary cell performance (ability to achieve high energy density and good efficiency since the amount of charge relative to volume can be made greater, etc.), the cell is not practical, because La is very expensive and so the cost of the manufactured cell is high. Further, the electrochemical capacity of the manufactured cell becomes low at room temperature since the hydrogen equilibrium pressure above LaNi.sub.5 is higher than atmospheric pressure at the temperature. It also results in the higher internal pressure of the cell than atmospheric pressure at room temperature which not only means that the cell case must be constructed strongly but also causes some problems; a lack of safety, a loss of the electrochemical capacity and self-discharge because of leakage of hydrogen gas.
There are examples of use of Mm (mischmetal: a mixture of rare earth elements containing 40-50 wt % of Ce and 20-40 wt % of La), which is much cheaper than pure lanthanum, as the rare earth component and of substitution of a portion of Ni with other elements in order to lower the equilibrium pressure. For example, the alkaline storage cell disclosed in Japanese Patent Disclosure No. 60-250558 consists of a negative electrode comprising a hydrogen absorbing alloy constituted by Mm, Ni, Co and Mk (Mm being a commercially available mischmetal (40 wt % La, 40 wt % Ce, 14 wt % Nd, 4 wt % Pr and other elements) and Mk at least one of the elements selected from the group consisting of Al, Sn, Cu, Fe, Mn, Cr, Mo, V, Nb, Ta, Zn and Mg), a positive electrode, a separator and an alkaline electrolyte.
A secondary cell which employs such a hydrogen absorbing alloy using conventional Mm as mentioned above as an essential material to the negative electrode can be manufactured at comparatively low price. However, although it keeps a large capacity and no production of gaseous hydrogen is observed initially, there are considerable restrictions on use of the cell since its life as a cell (the number of cycles of charging and discharging until the discharged capacity decreases below a prescribed quantity and the cell is no longer serviceable for practical purposes) is finished in a small number of charge-discharge repetitions. For example, with a cell capable discharge 100% of the charged capacity in the initial period, the discharged capacity decreases to 80% after about 100 charge-discharge cycles and to about 50% after about 150 charge-discharge cycles, which makes it difficult to apply such a cell to practical use. This is the result that the hydrogen absorbing alloy constituting the negative electrode is chemically unstable in an electrolyte in charge-discharge cycles, and therefore the chemical stability of the ally must be improved in order to increase the cycle life of the cell.